Plant ecology

History

After the glaciers in the UK retreated about 12000 years ago, a
tundra-like vegetation developed on the glacial till and loess deposits
covering the eroded limestone platforms. As climate improved, woodland
cover became established on the newly formed soil. This was mainly
birch and hazel to start with, followed by larger, longer-lived
trees such as oak and ash.

About 4000 years ago,
the effect of humans on the landscape started to outweigh the effect
of natural processes. Tree felling to clear land for crops and grazing
animals meant that areas which were originally wooded became less
so. It is known that some areas of limestone pavement which are
currently bare of woodland cover such as Malham, were once wooded.
Other areas of limestone pavement were not as badly affected by
grazing and soil loss and stayed wooded. The majority of wooded
pavements are found at low elevation around Morecambe Bay, although
there are some small areas of wooded pavement in the Yorkshire Dales,
Durness in Scotland and in North Wales.

Grazing animals accelerated
the process of soil loss from most upland pavement surfaces, and
vegetation cover became increasingly fragmented. Woodland plants
were unable to find a niche on the surface of the pavement where
the soils were thin or non-existent, and retreated into the grikes
between the blocky clints where there was deeper soil, shade, humidity
and protection from grazing.

This is why it is possible
to find typical woodland species such as honeysuckle (Lonicera
periclymenum), dogs mercury (Mercurialis
perennis), herb Paris (Paris quadrifolia)
and angular Solomon’s seal (Polygonatum
odoratum) in upland areas which are otherwise surrounded
by tracts of grassland and moorland. This is particularly true for
many woodland plants which have poor dispersal mechanisms where
the seeds have no way of getting to where the plants are currently
found. This woodland vegetation is a relic of the original plant
cover, and one of the things that makes limestone pavement so unusual.

Current
conditions

The clearance of woodland and the emergence of bare limestone clints
created conditions suitable for a range of plants which are characteristic
of open situations such as grassland or moorland. Where clints are
covered by a thin layer of soil, plants typical of the surrounding
limestone grassland will be found, for example, blue moor grass
– limestone bedstraw (Seslaria caerulea-Galium
sterneri) CG9 NVC communities. These plants are tolerant
of drought and can grow on the most skeletal of soils and include
thyme (Thymus polytrichus), sheeps
fescue (Festuca ovina) and common bent
(Agrostis capillaris).

Water collects in solution
pans and in these fen and flush vegetation such as rushes (Juncus
spp.) occur. Willow (Salix spp).
has been known to grow in some of the larger solution features on
limestone pavement.

Common plants
found on limestone pavement

The diversity of plants found on limestone pavements has a direct
relationship with the large number of ecological niches or habitats
found on a pavement. There are a number of plants always found on
limestone pavements throughout their geographic range in Britain.
Ward and Evans (1975) and Natural England (1995) concentrated surveys
on the plants associated with the grikes. These surveys showed that
the most frequent species found in the grikes were herb Robert (Geranium
robertium), maidenhair spleenwort (Asplenium
trichomanes), dog’s mercury, harts-tongue fern (Phyllitis
scolopendrium), wall-rue (Asplenium
ruta-muraria), male fern (Dryopteris
felix-mas), common dog violet (Viola
riviniana) and wall lettuce (Mycelis
muralis). The high proportion of ferns commonly found is
down to the shady, humid conditions in grikes.

Rare plants

There are a number of nationally scarce plant species associated
with limestone pavements which give them a high conservation value.
These are listed in the table below.

Some of these plants are barely found anywhere else apart from limestone
pavements. The best example of this is rigid buckler fern of which
85% of the UK population is found on limestone pavements.

A list of many of the
plants associated with limestone pavements can be found here with
an assessment of the types of pavements that they are found on.

Wooded limestone
pavement

Wooded pavements generally
have a covering of mosses and other bryophytes under a canopy of
trees. The limestone pavement patterning is most often hidden by
the growth of bryophytes and leaf litter. Trees are rooted in deep
patches of soil or in the grikes, with natural clearings showing
exposed areas of limestone pavement.

The main tree species
found on wooded pavements are ash (Fraxinus
excelsior) and sycamore (Acer pseudoplatanus)
with some oak (Quercus spp.), and wych
elm (Ulmus glabra). Some pavements
have a significant amount of yew (Taxus baccata),
and others occasionally, have small leaved lime (Tilia
cordata). Understorey species include spindle (Euonymus
europaeus), buckthorn (Rhamnus catharticus), hazel (Corylus
avellana)and hawthorn (Crataegus monogyna).
Many areas of wooded pavement have been coppiced in the past. Transition
areas between woodland and clearing support the greatest diversity
of plants with many of the species found on the open pavements also
found here.

Some very old trees were
once found within wooded pavement where they became established,
but have now found themselves on open pavement as cover has decreased.
Their establishment took place on deeper soils, their roots penetrated
cracks and hollows in the limestone, and they now find themselves
in a stressful environment. These trees, some of which are known
to be hundreds of years old, may become ‘bonsai’ in
form due to shoot die-back and browsing by deer and rabbits. Their
contorted, dwarfed forms are frequently grazed back into the grikes.
One particular tree on Gaitbarrows NNR has visibly shrunk in the
25 years between two monitoring photographs taken by Natural England
staff.

A reasonably comprehensive
list of plant species commonly found on limestone pavements can
be found here

The ecology of
grikes, runnels, pits and pans

Grikes

Grikes provide sheltered humid and shady conditions where climate
is more moderate than in the open. Grikes also provide a habitat
for species that are intolerant of grazing. The width and depth
of grikes varies a lot. Most grikes are between 1 and 2m deep, but
some have been measured as much as 6m in depth. Width can be anything
from a few mm to up to 500mm. Deep grikes are considered to be those
which are more than twice as deep as they are wide. It is only these
deep grikes which provide the protection from grazing and extremes
of climate which plants such as bloody crane’s-bill and angular
Solomon’s-seal need to survive.

Woodland floor plants
dominate the flora found in shallower grikes, and in the deepest
grikes, ferns such as hart’s tongue fern can be found as these
plants can tolerate the shadiness at depth.

Pits and pans

Pits and pans are the solution hollows associated with limestone
pavement. Isolated islands of vegetation on clints will often have
pits and pans developing underneath. The deeper soil here means
that vegetation can survive for longer than in runnels. However,
the isolated nature of the patch of vegetation means that eventually
the soil and plants will shrink until there is a point where all
will be lost through desiccation, wind erosion and disturbance.
The final phase of this sequence is an open pan which holds water
after rain with possibly an algal community being supported.

In pits and pans which
still have soil, an ephemeral fen community can become established
including plants such as Common Yellow sedge (Carex
viridula), articulated rush (Juncus
articulatus), butterwort (Pinguicula
vulgaris) and birds-eye primrose (Primula
farinosa). Pits and pans which have lost much of their soil
depth will often hold a community of drought tolerant plants such
as the stonecrops (Sedum spp). Much
of the limestone pavement in England is suffering from the advanced
stages of vegetation retreat with only isolated areas of soil cover.

Runnels

Most runnels have no vegetation, but where a soil cover is found,
the plants supported are limestone grassland type plants. These
include fescues, blue moor grass and wild thyme. Dropwort (Filipendula
vulgaris), flea sedge (Carex pulicaris)
and hairy violet (Viola hirta)can also
be found growing in some runnels.

The runnels keep their
soil and vegetation for longer than the clint tops as plant cover
retreats, and the soil in the runnels is the last remnants of the
post-glacial cover that would have been found right the way across
the limestone. Soil in the runnels is prone to desiccation as roots
of the plants cannot penetrate the limestone surface. When the soil
dries out, it can be easily dislodged or blown away. This is one
of the mechanisms by which the soil cover on limestone pavements
is reduced.